Jar tools are extensively used in the oil drilling industry to dislodge drill bits, drill string sections, or other well tools trapped in well bores. An example of such a jar tool is disclosed in U.S. Pat. No. 3,949,821 (Raugust) entitled DRILL STRING JARRING AND BUMPING TOOL WITH PISTON DISCONNECT and owned by the applicant. U.S. Pat. No. 3,949,821 discloses a jar tool which has been in satisfactory use for a number of years.
Problems have remained with the patented apparatus, however, which have resulted in improvements being made which are the subject of this application.
U.S. Pat. No. 3,949,821 discloses a jar tool having a spline mandrell and an outer housing, the mandrell and the housing being able to move longitudinally relative to each other. A knocker is connected to the mandrell and it collides with an anvil which is connected to the housing. The collision or impact between them causes the jar on the drill string and it should be appreciated that the impact is violent and the operating conditions under which the tool is used are extreme. This impact is created by the sudden release of entrapped hydraulic fluid which is allowed to rush by a piston assembly travelling from a relatively narrow diameter of a hydraulic cylinder into a relatively larger diameter where the fluid flow past the piston is largely unimpeded.
To actuate the piston assembly, a piston actuator is used. This piston actuator includes resilient fingers extending from the actuator and towards the piston. The fingers engage a resilient part on the piston assembly when contact is made and provide a "pulling" action on the piston assembly to return it to its home position following the jar. The technique of using these fingers and the necessary costs entailed in using them and complementary parts on the piston assembly is expensive and complicated.
Further, it was considered desirable to improve the metering system. The use of the metering system, required fluids that, although having viscosity characteristics more constant under different operating temperatures, did not have desirable lubricating qualities. The metering system could be improved by selecting temperature compensable valves within the metering system and pressure relief valves. The metering and pressure relief valves may be used in various combinations to achieve desirable operating characteristics.
The use of a metering system which allows for the changes in fluid viscosity and density as the temperatures within the drill hole vary, allows for the use of more desirable fluids from a lubriction view point.